Production of volatile sulfides from freshwater algae and implications for the environment

Abstract

<p>Dimethyl sulfide (OMS) is the major volatile sulfide (VS) species of biogenic origin released to the atmosphere. Large amounts of OMS can originate from the decomposition of the metabolic precursor Dimethylsulfoniopropionate (DMSP) in marine algae. Quantitatively, the emissions of OMS and other VS to the atmosphere are in the same order of magnitude as atmospheric sulfur from man-made pollution. Therefore the study of OMS and other VS has a significant relevance to atmospheric chemistry.</p> <p>Early studies on OMS focused on the marine environment and coastal waters. The presence of other biogenic species such as H₂S, COS, CS₂, CH₃SH (MeSH), CH₃SSCH₃ (DMDS) was also detected in various types of environments including salt ponds, meromictic lakes, soils, freshwaters.</p> <p>The present work is a laboratory-oriented study on the processes of formation of VS from freshwater algae. The approach undertaken is threefold: 1) Establish the presence and/or production of OMS and possibly other VS from algae cultures and surrounding lake waters. 2) Identify some key parameters influencing the formation of VS from freshwater algae. 3) Estimate the atmospheric contribution of VS from some freshwater algae and lakes.</p> <p>This study showed that all species of algae and all natural waters contain OMS in amounts comparable to marine waters. However, in contrast to marine waters, OMS generally constitutes only 5 to 25% of all the sulfides, whereas MeSH usually predominates (40-80 %). H₂S+COS, CS₂ and DMDS are commonly present, whereas other species are rarely detected.</p> <p>Changes in sulfate concentration had 115 significant effect on VS production by algae, but there was a threshold level of about 2 mg/L SO₄ for VS production. Cysteine also caused no significant change in VS production, but a small influence was observed with the addition of methyl donors. Methionine holds a key part in formation of methylated sulfides: additions provoked an increase of MeSH and DMDS production by up to 2 or 3 orders of magnitude.</p> <p>Calculations suggest that VS from freshwater algae contribute about 115 tons S/year in Lake Ontario. This estimate is quite variable, but it is small compared to other sulfur inputs, including pollution.</p>